High frequency radio aerials



13% m Q 9mm EWUE Jan. 6, 1959 M R ET AL 2,867,801

HIGH FREQUENCY RADIO AERIALS Filed se i. 14, 1953 INVENTORS @1174 7mm W/ ATTORNEYS United States Patent 9 i HIGH FREQUENCY RADIO AERIALS Peter Frederick Mariner, Elstree, and Charles Alexander Cochrane, Potters Bar, England, assignors to Elliott Brothers (London) Limited, London, England, a British company Application September 14, 1953, Serial No. 379,858

12 Claims. (Cl. 343-756) This invention relates to high frequency radio aerials and is concerned more particularly with improvements in or modifications of the invention described and claimed in the specification of copending application Serial No. 271,679, dated February 15, 1952, issued February 28, 1956, as Patent No. 2,736,895.

in view of the relatively narrow width of the beam radiated from a high frequency radio aerial, it is frequently necessary to scan the beam so that it effectively covers a larger solid angle than corresponds to its own width while yet retaining the directivity inherent in a narrow beam. The means hitherto employed for scanning such beams are subject to certain disadvantages, such as the necessity for providing rotating joints in the coaxial transmission line or waveguide feeding the energy to the aerial, the difficulty of suitably mounting and rotating the reflector element of the aerial system, or de terioration of the radiation polar diagram due to the relative movement between the feeding and focussing elements.

The object of the present invention is to provide an improved aerial system having means for scanning the radiated beam which shall not be subject to the disadvantages mentioned and shall afford certain added advantages.

In the specification of the earlier application referred to above there is described and claimed an invention broadly consisting of a high frequency radio aerial comprising a primary source of radiation, a main reflector and an auxiliary reflector arranged to reflect radiation from the primary source on to the main reflector for re-reflection into free space as an emergent beam, the auxiliary reflector being disposed at least partially in the path of this beam, characterised by the arrangement of the primary source to radiate energy polarised in a given plane, the construction of the auxiliary reflector so that it will reflect incident energy which is polarised in the given plane but will transmit substantially all such energy which is polarised in a plane at right-angles to the given plane, and the association with the main reflector of means which will rotate through substantially 90 the plane of polarisation of any radiation reflected thereto by the auxiliary reflector.

The auxiliary reflector, in accordance with another feature of the earlier invention, is arranged to perform a collimating function such that the radiation incident upon the main reflector, and the polarisation-rotating means associated therewith, will be a plane wave the direction of propagation of which is fixed in relation to the axis of the focussing device. The polarisationrotating means associated with the main reflector is composed of a set of electrically conducting layers disposed with their faces parallel with the direction of propagation of the radiation incident thereon and spaced apart by a distance which is less than half a wave-length of this radiation, the layers being arranged at 45 to the direc- 2,867,801 Patented Jan. 6, 1959 tion of polarisation of the electric vector of this radia tion and the distance between the front edges of the layers and the reflecting surface of the main reflector being equal to a quarter of a wave-length in the dielectric material employed of the radiation, or to any odd multiple of this length. The polarisation means associated with the main reflector will be referred to herein, for convenience, as a half-wave plate.

Now, according to the present invention, a high frequency radio aerial embodying the features set forth in the immediately preceding two paragraphs is modified to permit scanning of the emergent beam by mounting the main reflector together with its associated half-wave plate so that they may be inclined in relation to the axis of the auxiliary reflector or focussing device.

The waveguide, or coaxial transmission line, feeding the aerial, the focussing device and any other elements such as correcting plates may thus remain stationary while the main reflector and its half-wave plate are inclined to a greater or lesser degree to cause deflection of the plane wave incident thereon through whatever angle is required for scanning purposes.

Since the moving element of the aerial is acting on a plane wave no aberrations are produced in scanning.

Errors due to the incidence of the energy upon the half-wave plate in directions other than the normal to the front face of the plate may, if desired, be prevented by constructing the half-wave plate so that the incident energy is constrained to pass therethrough in a direction perpendicular to the face of the plate, for example, by employing an egg-box arrangement of the electrically conducting layers in place of the simple parallel arrangement referred to above.

Errors due to rotation of the polarisation direction of the half-wave plate in relation to the polarisation direction of the incident energy may be obviated by suitably disposing the axes about which the main reflector (with the half-wave plate) is adapted to rotate. For example, where the main reflector is supported in a gimbal system and the half-wave plate is of parallel-layer construction, the axis of the inner pivots should be parallel with the front edges of the layers.

In an aerial according to this invention the means for scanning the emergent beam is independent of the feeding and focussing elements and these may be made part of the structure for supporting the aerial. For example, where the aerial is required to be encased in a radome, the focussing device or auxiliary reflector may constitute the front portion of the radome, in which case aberrations due to the obtrusion of radome structure into the path of the emergent beam are avoided.

The scanning effect is obtained on the reflecting mirror principle and the main reflector is thus required to move through only approximately half the angle through which the beam is to be scanned. Since, also the moving parts are relatively light in weight, the mechanical problems involved are small.

In order that the invention may be more clearly understood one form thereof will now be described by way of example with reference to the accompanying drawing which is a perspective view of one form of radio aerial according to this invention.

The high frequency radio aerial illustrated comprises a plane main reflector 1, shown as a circular metal plate. and a waveguide 2 of rectangular cross-section extending axially through an aperture (not shown) at the centre of the reflector 1. The waveguide terminates, at a distance in front of the reflector 1, in a horn 3 which is arranged to radiate high frequency energy, fed thereto through the waveguide 2, towards an auxiliary mirror 4, the plane front face 4a of which is disposed parallel with the reflecting surface of the main reflector 1. The mirror 4 is composed of a considerable number of flat strips 5 of an electrically con-ducting material, such for example as metal foil or a coherent layer of metal particles, which are arranged side-by-side in parallel relationship and are spaced from each other by interposed strips 6 of a solid dielectric material, such as, for example, an expanded polyvinyl chloride polymerisation product. The strips 5 are of uniform width and have their longer edges suitably curved (as shown in broken lines for three only of the strips) so that those edges disposed nearer to the horn 3 are all contained in a paraboloidal surface. 6 of dielectric material have their front longitudinal edges all contained in the face 4a and may be either of varying widths (as shown) so that their rear longitudinal edges are also all contained in the paraboloidal surface containing the edges of the strips 5, or all of the same constant width so that their rear edges are all contained in a plane containing the rear edge of the mirror 4. The strips 6 are all of the same thickness which is such that the spacing between adjacent strips 5 is somewhat less than half a wave-length of the high frequency energy being fed through the waveguide 2. The wider faces of all the conducting strips 5 are located in planes parallel with the planes containing the shorter or side walls 2a of the waveguide 2, from which it follows that the longitudinal edges of the strips 5 which are located in the paraboloidal surface all lie parallel to the direction of polarisation of the electric vector of the radiation from the born 3.

Threaded on the waveguide 2 between the main reflector 1 and the horn 3 is a half-wave plate 7 which consists of a circular disc built up from layers 8 of an electrically conducting material, e. g. metal foil, interleaved with layers of a dielectric material, for example, an expanded polyvinyl chloride polymerisation product. The layers 8 are set edgewise to the circular faces and parallel with each other, being spaced apart by a distance which is less than half a wave-length of the high frequency energy supplied through the waveguide 2. Moreover, the orientation of the half-wave plate 7 is such that the planes containing the wider faces of the layers 8 form dihedral angles of 45 with the planes containing the wider faces of the strips 5. The thickness of the halfwave plate 7 is suflicient to ensure that all or substantially all, the radiation which is incident upon a flat face of the plate and which has the electric vector polarised parallel to the layers 8 will be reflected, and the front face of the plate 7 is spaced from the reflecting surface of the main reflector 1 by a distance equal to a quarter of a wavelength, in the dielectric material employed, of the high frequency energy supplied through the waveguide 2 or to any odd multiple of this length.

The main reflector 1 and the half-wave plate 7 are secured together to form a reflecting unit and the waveguide 2 extends with clearance through a central aperture 9 formed therethrough. The reflecting unit is mounted on trunnions 10 which are rotatably received in gimbal bearings 11 in a square frame 12 so that the reflecting unit may rotate about an axis parallel with the front edges of the layers 8. The frame 12 is itself provided with a pair of trunnions 14 so supported in bearings 15 provided in a fixed support, part of which is shown at 16, that the frame 12 together with the reflecting unit may be rotated about an axis normal to that about which the reflecting unit may rotate Within the frame 12. In order to produce the desired scanning effect it is necessary to oscillate the reflecting unit about one or both axes and for this purpose suitable means must be provided as will be readily understood by those skilled in the art. For the purposes of the present example there is shown in the drawing a motor 17 attached to the frame 12 and arranged to drive a cam 18 on which bears a finger element 19 secured to one of the trunnions 10 and urged into engagement with the cam 18 by a spring 20 secured at one end to the finger The strips element 19 and at the other end to the frame 12. There is also provided a reversible motor 21 mounted on the support 16 and directly connected to one of the trunnions 14 to oscillate the frame 12 about the axis of the trunnions 14.

In use of the aerial described, high frequency energy of the appropriate wave-length is fed to the aerial through the waveguide 2 and is radiated by the horn 3 towards the paraboloidal surface of the auxiliary mirror 4, this radiation being polarised so that the direction of the electric vector of the radiation is parallel with the planes containing the wider faces of the conducting strips 5. Owing to the spacing of these, all, or substantially all, of the incident radiation is reflected back towards the half-wave plate 7, the reflected radiation being concentrated into a parallel beam due to the collimating action of the auxiliary mirror 4. When this reflected radiation meets the front surface of the half-wave plate 7, the direction of its electric vector is at 45 to the edges of the conducting layers 8. Owing to the spacing of these layers 8, that wave component polarised parallel to the layers 8 is reflected forwards to the auxiliary mirror 4. However, that wave component polarised perpendicularly to the layers 8 passes between these layers to the reflecting surface of the main reflector 1 and is there re-reflected to pass once again between the conducting layers 8, this time towards the auxiliary mirror 4. The polarisation of the re-reflected wave component which has traversed the plate 7 has now been rotated through 180. Hence, the high frequency energy composed of the two re-reflected components travelling towards the auxiliary mirror 4 has had its plane of polarisation rotated through with respect to that of the high frequency energy radiated from the horn 3. Since the direction of polarisation of the electric vector of the re-reflected radiation is now perpendicular to the planes containing the wider faces of the conducting strips 5 of the auxiliary mirror 4, the rereflected radiation will pass straight through the mirror 4 which will have little or no effect upon the beam emerging from the mirror 4.

It will be seen that operation of the motor 17 will cause the cam 18 to rotate to oscillate the finger element 19 and the reflecting unit 1 and 7 about the axis of the trunnions 14, an oscillation of the unit through a total angle of 10 being usually sufficient. At the same time the reversible motor 21 is used to oscillate the frame 12 about the axis of the trunnions 14, the speed of the motors 17 and 21 being adjusted relative to each other to produce the desired compound motion and resultant scanning effect of the reflecting unit 1, 7.

It will be understood that the feature of the invention may be utilised in a high frequency radio aerial arranged to collect high frequency energy from free space and to supply this energy to a receiver, the main reflector unit 1, 7 being mounted for rotation about at least one axis normal to that of the auxiliary reflector and means being provided to rotate the main reflector about this axis to cause it to be directed into free space in a predetermined direction.

What we claim is:

1. A high frequency radio aerial comprising a primary source emitting radiation polarised in a given plane, an auxiliary reflector arranged to be illuminated by said radiation and constructed as a collimator to reflect with a plane wave-front substantially the whole of the incident radiation polarised in said given plane but to permit free passage therethrough of substantially all radiation incident thereon which has its plane of polarisation at substantially 90 to said given plane, a main reflector arranged to be illuminated by radiation reflected from said auxiliary reflector and constructed to re-reflect said reflected radiation as an emergent beam polarised in a plane substantially at 90 to said given plane and means for oscillating said main reflector about at least one axis normal to the axis of the main reflector to cause the radiation reflected from said main reflector to follow a predetermined path.

2. An aerial according to claim 1, wherein the auxiliary reflector comprises a set of electrically conducting layers disposed parallel with the direction of the polarisation of the electric vector of the radiation emitted by the primary source and spaced apart by a distance less than half a wave-length of said radiation.

3. An aerial according to claim 1, wherein the main reflector comprises a totally reflecting surface faced with a set of electrically conducting layers disposed at 45 to the direction of the polarisation of the electric vector of the radiation emitted by the primary source and parallel with the direction of propagation of said radiation, the spacing between the layers being less than half a wavelength of said radiation.

4. A high frequency radio aerial comprising a pri mary source emitting radiation polarised in a given plane, an auxiliary reflector arranged to be illuminated by said radiation and constructed as a collimator to reflect with a plane wave-front substantially the whole of the incident radiation polarised in said given plane but to permit free passage therethrough of substantially all radiation incident thereon which has its plane of polarisation at substantially 90 to said given plane, a main reflector arranged to be illuminated by radiation reflected from said auxiliary reflector and constructed to re-reflect said reflected radiation as an emergent beam polarised in a. plane substantially at 90 to said given plane and means for oscillating said main reflector about two axes normal to each other and normal to the axis of the main reflector to cause the radiation reflected from said main reflector to follow a predetermined path.

5. A high frequency radio aerial comprising a primary source emitting radiation polarised in a given plane, an auxiliary reflector arranged to be illuminated by said radiation and constructed as a collimator to reflect with a plane wave-front substantially the whole of the incident radiation polarised in said given plane but to permit free passage therethrough of substantially all radiation incident thereon which has its plane of polarisation at substantially 90 to said given plane, a frame pivotally mounted in a relatively fixed structure, means for oscillating said frame about a first axis normal to the axis of said auxiliary reflector, a main reflector pivotally mounted in said frame for oscillation about a second axis normal to both said first axis and said axis of said auxiliary reflector and arranged to be illuminated by radiation reflected from said auxiliary reflector and constructed to re-reflect said reflected radiation as an emergent beam polarised in a plane substantially at 90 to said given plane and means for oscillating said main reflector about said second axis to cause the radiation reflected from said main reflector to follow a predetermined path.

6. A high frequency radio aerial comprising a primary source emitting radiation polarised in a given plane, an auxiliary reflector arranged to be illuminated by said radiation and constructed as a collimator to reflect with a plane wave-front substantially the whole of the incident radiation polarised in said given plane but to permit free passage therethrough of substantially all radiation incident thereon which has its plane of polarisation at substantially 90 to said given plane, a frame pivotally mounted in a relatively fixed structure, a main reflector pivotally mounted in said frame and comprising a totally reflecting surface faced with a set of electrically conducting layers disposed in planes lying at 45 to the direction of the polarisation of the electric vector of the radiation emitted by the primary source and parallel with the direction of propagation of said radiation, the spacing between the layers being less than half a wave-length of said radiation, means for oscillating said main reflector about a first axis normal to the axis of said auxiliary reflector and parallel with the planes of said conducting layers and means for oscillating said frame about a second axis normal to said first axis and to the axis of said auxiliary reflector to cause the radiation reflected from said main reflector to follow a predetermined path.

7. A high frequency radio aerial comprising a receiver for radiation, a main reflector adapted to collect high frequency energy from free space and an auxiliary reflector arranged to re-reflect onto said receiver radiation reflected from said main reflector, said auxiliary reflector being disposed at least partially in the path of said incident-energy, in which the auxiliary reflector is constructed so that it will permit free passage to said main reflector of incident energy which is polarized in a given plane but will reflect substantially all energy incident thereon which is polarized in a plane at right-angles to said given plane, means associated with the main reflector adapted to rotate through substantially the plane of polarization of any radiation polarized in said given plane which is incident on said main reflector, and means for rotating said main reflector about at least one axis normal to the axis of the main reflector.

8. An aerial structure comprising an element capable of receiving and transmitting high frequency electromagnetic energy, an auxiliary reflector constructed to permit the free passage therethrough of substantially all high frequency electromagnetic energy incident thereon polarised in a given plane and to reflect substantially all such energy polarised in a plane at right-angles to said given plane, a main reflector arranged to rotate through 90 the plane of polarisation of any high frequency energy incident thereon polarised in said given plane and means for oscillating said main reflector about at least one axis normal to the axis of the main reflector, said element, said auxiliary reflector and said main reflector being so disposed relative to each other as to provide a path between said element and said auxiliary reflector and said auxiliary reflector and said main reflector along which energy may travel in one direction from said element to said main reflector and in the direction opposite to said one direction from said main reflector to said element.

9. An aerial structure comprising an element capable of receiving and transmitting high frequency electromagnetic energy, an auxiliary reflector and a main reflector, said element, said auxiliary reflector and said main reflector being so disposed relative to each other as to provide a path along which energy may travel in either of two opposed directions between said element and said auxiliary reflector, said path comprising one part between said element and said auxiliary reflector and two parts extending respectively in substantially opposite directions between said auxiliary reflector and said main reflector, said auxiliary reflector being constructed to permit the free passage therethrough of substantially all high frequency electromagnetic energy incident thereon polarised in a given plane and to reflect substantially all such energy polarised in a plane at right-angles to said given plane and said main reflector being arranged to rotate through 90 the plane of polarisation of any high frequency energy passing thereto from said auxiliary reflector, and means for oscillating said main reflector about at least one axis normal to the axis of said main reflector.

10. An aerial structure according to claim 9, wherein the reflecting surface of said auxiliary reflector is of paraboloidal form and said element is disposed substantially at the focus thereof and substantially on the axis of said main reflector.

11. An aerial structure comprising an element capable of receiving and transmitting high frequency electromagnetic energy, an auxiliary reflector constructed to reflect substantially all high frequency electromagnetic energy incident thereon polarised in a given plane and to permit the free passage therethrough of substantially all such energy polarised in a plane at right-angles to said given plane, a main reflector arranged to rotate through 90 the plane of polarisation of any high frequency energy incident thereon polarised in said given plane and means for oscillating said main reflector about at least one axis normal to the axis of the main reflector, said element, said auxiliary reflector and said main reflector being so disposed relative to each other as to provide a path between said element and said auxiliary reflector and said auxiliary reflector and said main reflector along which energy may travel in one direction from said element to said main reflector and in the direction opposite to said one direction from said main reflector to said element.

12. An aerial structure comprising an element capable of receiving and transmitting high frequency electromagnetic energy polarised in a given plane, an auxiliary reflector comprising a set of electrically conducting layers disposed parallel with the direction of the polarisation of the electric vector of said energy polarised in said given plane and spaced apart by a distance less than half a wavelength of said energy such that said auxiliary reflector will reflect substantially all such energy incident thereon polarised in said given plane and will permit the free passage therethrough of substantially all the high frequency electromagnetic energy incident thereon polarised in a plane at right angles to said given plane, a frame pivotally mounted in a fixed structure, a main reflector pivotally mounted in said frame and comprising a totally reflecting surface faced with a set of conducting layers disposed in planes lying at to the said direction of polarisation of said electric vector, the spacing between said conducting layers with which said totally reflecting surface is faced being less than half a wave-length of said energy, means for oscillating said main reflector about a first axis normal to the axis of said auxiliary reflector and parallel with the planes of the conducting layers of said main reflector, and means for oscillating said frame about a second axis normal to said first axis and the axis of said auxiliary reflector, said element, said auxiliary reflector and said main reflector being so disposed relative to each other as to provide a path between said element and said auxiliary reflector and said auxiliary reflector and said main reflector along which energy may travel in one direction from said element to said main reflector and in the direction opposite to said one direction from said main reflector to said element.

References Cited in the file of this patent UNITED STATES PATENTS Germany Nov. 28, 1938 

